1. Field of the Invention
This invention deals with a pulp treatment apparatus and method to effect automatic control of pulp consistency within a predetermined range by control of the effluent discharge of a controlled drainage thickener to thereby vary the consistency of a pulp suspension being fed from the thickener to a screw type pulp press.
2. Description of the Prior Art
Dewatering devices used in the paper industry generally are based on (1) drainage through a mat aided by a vacuum, (2) compression and shear, or (3) centrifugation. Thickening of suspensions can be grouped into several solid ranges, namely, 0.5 to 3%, 3 to 10%, and 10 to 40%. A Decker thickens from about 1% to about 10% and a continuous centrifuge thickens from about 4% to about 20%. The throughput and the efficiency of these devices are typically a function of the material's dewatering properties. All of these devices, however, have little or no control over the quantity of liquid expressed from them. Generally speaking, either consistency or level controls are utilized in an attempt to achieve a certain consistency range so that the consistency is appropriate for the most efficient operation of apparatus following the dewatering device.
A thickener is unlike a pulp press in that it acts more in the nature of a screen. Stock is introduced into the thickener by means of an inlet pipe under feed pressure. A pressure differential is produced across the screen by virtue of the feed pump and the back pressure from the effluent discharge. The magnitude of this pressure differential is controlled by a valve regulating the effluent flow rate. Pulp is forced against the screen and partially dewatered by virtue of the pressure differential. Periodically the pulp is wiped from the surface of the screen by the flights of a rotating screw. The clearance between the flight and the basket is typically about 0.030 inches. At lower rates, the screw also contributes to pulp flow. The flight to screen clearance is maintained within the smallest possible distance to insure sufficient wiping of the screen surface. The tight clearance is especially important with pulp in the operating consistency range of 2 to 12%.
A pulp press, on the other hand, dewaters stock by compacting or squeezing the pulp to force the water out. In a typical screw type press such as the Beloit-Jones Horizontal Pressmaster, the central shaft of the screw becomes larger in diameter from the inlet to the outlet ends and the pitch of the flights changes. The press has some back pressure on the inlet end to prevent slippage of the stock around the screw as it is being compacted to force the water out. The controlled back pressure usually takes the form of a column of stock above the inlet.